1. Field of the Invention
This invention relates generally to methods for the formation of ohmic contacts on bodies of semiconducting materials and more particularly to the formation of low-resistance ohmic contacts on semiconducting oxides. The term "ohmic contact" is used herein to refer to a metallic (metal or metal-alloy) electrode whose electrical resistance is constant in an applied electric field. The term "low-resistance ohmic contact" is used herein to refer to ohmic contacts whose resistance is only a small percentage of that of the typical junction device. The term "semiconducting oxide" is used to refer to wide-band-gap semiconducting materials having oxygen as a constituent--as, for example, barium titanate (BaTiO.sub.3), lithium niobate, and zinc oxide. The semiconducting oxide may be of the n- or p-type and may or may not contain an electrical junction.
2. Problem
The utilization of n-type semiconducting-oxide devices has been limited by the lack of a relatively simple, rapid, and reproducible method for providing the oxide with high-quality metallic contacts for the attachment of electrical leads. Various conventional techniques (e.g., vapor-deposition) produce satisfactory contacts on other semiconductor materials but when applied to n-type semiconducting oxides they often result in contact resistances which are unsuitably high. Such techniques are believed to be deficient because they do not disrupt a space charge layer present on the surface of the oxide material. That is, it is believed that absorbed oxygen acceptor states at the oxide surface result in a depletion layer at the oxide-to-contact interface, creating a current barrier.
Low-resistance ohmic contacts may be formed on n-type semiconducting oxides by methods which entail mechanical or chemical disruption of the above-mentioned space-charge layer, but these methods are subject to significant disadvantages. For instance, ohmic contacts can be formed on the oxides by chemically depositing a layer of nickel and then heat-treating the layer. Unfortunately, that technique requires relatively complex equipment and presents waste disposal problems. Another conventional contact-forming technique comprises the deposition of metals such as gold and silver by flame-spraying; however, this requires relatively expensive equipment and is attended by health and safety problems associated with metal inhalation and noise. Another known contact-forming technique comprises rubbing the oxide surface with indium wetted with mercury or gallium. The resulting contacts are not highly uniform, however, and they age quickly at room temperature. The prior art also includes forming ohmic contacts by ultrasonically soldering indium-based alloys to the oxide. Unfortunately, the resulting contacts do not have as high a uniformity as desired, and they are useful only in the temperature range below about 300.degree. C.
U.S. Pat. No. 4,147,563, issued on Apr. 3, 1979, to J. Narayan and R. T. Young, discloses the use of laser pulses to diffuse a superficial layer of dopant material into a silicon substrate to form a p-n junction therein or to form silicide contacts. U.S. Pat. No. 4,181,538 issued on Jan. 1, 1980 to J. Narayan, C. W. White, and R. T. Young, discloses the use of laser-pulse annealing to improve the electrical properties of doped or undoped silicon substrates.